Pressure-sensitive adhesive (PSA) constructions such as labels, tapes, decals and the like are known in the art. For example, PSA label constructions are commonly used to apply a particular facestock having a specific nature of printing to an object or article. PSA label constructions typically comprise a release liner, a PSA layer disposed onto the liner, and a facestock laminated onto the PSA layer. This lamination may be formed by first coating or laminating the PSA to the liner, then laminating the facestock onto the PSA-coated liner; or alternatively by coating or laminating the PSA to the facestock, then the PSA-coated facestock onto the liner. The facestock is characteristically made from a web or sheet of paper, cardboard or plastic, which is printed on with information or other indicia either before or after it is laminated to the PSA and liner. A plastic facestock can be a single layer or multiple layers formed either by lamination or coextrusion. In a typical process of "converting" the facestock/PSA/liner laminate, the facestock is printed on the exposed facestock surface, die-cut down to the liner surface to outline the label shape, and the waste material between the labels (matrix) is stripped out. The PSA label facestock and adhesive is then adhered to a substrate surface by separating the label from the liner and causing the PSA layer of the label to come into contact with and bond to the substrate surface with the PSA layer providing the bonding force (as measured by a peel test). In the most popular labeling process, the label is separated from the liner by bending the liner back over a peel-plate, whereupon the label is sufficiently stiff to cause the label to continue on a straight path toward the desired substrate surface overcoming the release force between the label adhesive and the silicone coated release liner. Peel-plate dispensing is generally done at room temperature.
As used in this patent application, "separation" refers to removal of the label from the liner, "application" refers to adhesion of the label to the substrate surface, and "dispensing" or "dispensability" refers to the combined steps of separation and application. "Peel-plate dispensing" as used herein, denotes the use of a peel-plate, sharp edge, or other similar device having a small radius of curvature, in the separation of liner from label.
The term "faceless" pressure sensitive adhesive constructions is used in the present application to refer to the very thin (e.g., less than 1 ml, preferably 0.1 to 0.5 or 0.6 mils), and to contrast the manufacturing methods disclosed herein from conventional PSA label construction manufacturing methods. As discussed above, in conventional manufacture of PSA label materials, a self supporting preformed web or sheet is laminated to a PSA ("preformed" means that the facestock had been formed into a self supporting web or sheet in a previous manufacturing process, and in the case of a liquid or molten facestock material, that the material has been dried or hardened). Faceless constructions of the present invention are formed by coextrusion of a film forming material (herein sometimes referred to as "FFM") and an adhesive whereby the facestock web or sheet and the adhesive are formed in situ.
In the manufacture and production of PSA constructions, a substantial amount of the overall cost involved is attributed to the material costs for the different material layers, e.g., the PSA and the facestock, be it paper, cardboard, or plastic, and this is one constraint on the selection of the type and thickness of the various layers. The layer thicknesses and layer materials for such conventional PSA constructions must also be selected to provide desired properties of convertibility, e.g., by conventional converting techniques such as by die-cutting and matrix-stripping; dispensability, e.g., by conventional dispensing equipment such as by peel-plate; and conformability, e.g., enabling the applied label to adhere to an irregular or deformable substrate surface without becoming detached or damaged.
It is known that the stiffness of a PSA construction will have an impact on its convertibility and dispensability. The stiffness of a label of a given material decreases as the label is made thinner. As a rule of thumb, as the construction stiffness is increased, so is convertibility and dispensability of the construction. However, the conformability of a PSA construction is known to decrease as the construction stiffness is increased. Thus, the objective stiffness for a PSA construction is a compromise between convertibility/dispensability, conformability, and cost. Too low a stiffness of the label will cause the label to go around the peel-plate with the liner. It is one objective of this invention to provide a minimal material label construction with enough stiffness to be dispensable using a peel-plate.
Prior art PSA constructions having a Gurley stiffness of at least and more commonly of at least 20 or greater are known and are described in U.S. Pat. Nos. 5,186,782; 5,516,393; 4,713,273; and 5,451,283. The '782, '393 and '283 patents exploited the idea that a proper differential between machine direction stiffness and cross dimension stiffness, with the latter being the lower of the two, could enable a heat-set film to be dispensed at high speeds, yet be suitable for flexible-film end uses. Such label film might exhibit acceptable overall conformability to flexible substrates even though the film has less inherent conformability than the then standard polymer labels based on polyvinyl chloride (PVC).
Specific end uses calling for highly-conformable PSA constructions include those where the label is to be adhered to a small-diameter contoured surface or irregular surface. In such an end use, unnecessary construction stiffness or rigidity could interfere with label's ability to conform and remain adhered to the underlying substrate surface. Additionally, these conventional PSA constructions are not manufactured in a most economically efficient manner.